deals with showing redirects to users that haven't paid for their PPPoE accounts. It can easily be adjusted for Hotspots (or even non-authenticated users) but will require a locally reachable web server.
Alternatively, if you're not using a Hotspot yet you can enable one with an announcement page and no way of logging in. If you are using a Hotspot you can switch HTML directories to an announcement one and log everybody out.
That isn't recommended if you are using PPPoE, they don't mix well with Hotspots and combining the two, while possible, isn't recommended by Mikrotik staff.